It all depends on what are the powers your attackers have. If your attackers can monitor the whole web, inject packets as he wish and control every computer in the world - there probably isn't any way to offer secrecy.

On the other hand, if we're talking about an adversary with a more limited capabilities - maybe he can access only 50% of the computers in the world and that cannot crack encryption at will - Than there are papers talking about theoretically sound systems that can give you anonymity with very high reliability.